The present invention is directed to a method for separating or purifying a substance to be separated/purified by crystallizing onto a cool surface from a molten mixture. The present invention is also directed to an apparatus for separating and/or purifying such a substance, with crystallization of the substance to be separated/purified from a mixture in a molten state taking place on a cooled surface in this apparatus.
Purifying or separating organic substances by crystallization from a molten mixture of the substances, has been very well known. The attainable efficiency of such separation can be determined from a phase diagram of the mixture of the substances in question. Most commercial melt-crystallizing processes make use of indirect cooling. The heat to be removed is then transferred through a partition, for instance, through a tube wall, to the coolant.
Crystallization may take place either in suspension or directly onto the cooled surface. A batch-type crystallizer is most usually involved in the latter case, where the accumulating crystallized deposit is removed by melting. Thus no new mixture to be crystallized can be introduced into the same apparatus at the same time. However, continuously operating crystallizers are also known, which are provided with cooling, melting and mixing apparatus. Examples of rotating melt crystallizers may be found, for example, in German patents Nos. 2,214,481 and 2,057,824.
A cylinder crystallizer is disclosed in German patent No. 2,214,481 in which the crystallizing space consists of the lowermost part of an annular space between two rotating cylinders, one inside the other. Both cylinders rotate about a horizontal axis. Melt to be treated is conducted into the annular space between the cylinders, this melt being crystallized at the lowermost point of the annular space to form a plug with the aid of coolers disposed outside the annular space. The crystallizing surface of the plug is continuously flushed by a flow of the melt being treated, while at the same time non-crystallizing components of the mixture and impurities are removed from the melt with the aid of a pipe extending into the annular space.